The present invention generally relates to a method and apparatus for monitoring aerosols or particulates in a fluid, and more specifically, to a method and apparatus for monitoring the lidar ratio for particulates or aerosols in the atmosphere.
A recent National Research Council panel report summarizes six independent lines of evidence supporting the hypothesis that direct (i.e., clear-sky) climate forcing due to the scattering and absorption of sunlight by anthropogenic aerosols is a major factor in global climate change. Visibility is similarly known to depend on scattering and absorption of light by atmospheric aerosols. A variety of aerosol measurements (as well as theoretical models) contribute to this evidence, but notably lacking is a physically meaningful contribution from elastically scattering lidar. Nevertheless, the potential contribution of this technology is enormous, given its exquisite precision, vertical resolution, and the relative ease of data acquisition. This potential has yet to be exploited because of difficulties in quantitatively and accurately relating the elastically scattered lidar signal to the aerosol parameters relevant to climate forcing and visibility.
Analogous to radar but operating at shorter wavelengths, a lidar instrument transmits pulsed laser radiation and measures what is backscattered by gases, particles, or other objects in the atmosphere. The return time of the signal corresponds to distance from the transmitter such that range-dependent information is acquired. The intensity of the signal depends on two quantities: (1) how effectively the laser radiation is backscattered at a specific location in the atmosphere; and, (2) how effectively the laser radiation is extinguished by the intervening atmosphere. Interpreting the lidar signal depends on an ability to separate these two quantitiesxe2x80x94local 180xc2x0 backscatter and optical depth over the entire range. It is this deconvolution of local backscatter and range-dependent optical depth, which is at the heart of the lidar retrieval challenge.
Following instrument calibration, a vertically pointing lidar provides a direct measurement of the quantity S(z), characterized by the following equation:                               S          ⁡                      (            z            )                          =                              A            ⁢                          xe2x80x83                        ⁢                          β              ⁡                              (                z                )                                      ⁢                          exp              ⁡                              [                                                      -                    2                                    ⁢                                                            ∫                                              z                        L                                            z                                        ⁢                                                                                            σ                          e                                                ⁡                                                  (                                                      z                            xe2x80x2                                                    )                                                                    ⁢                                              xe2x80x83                                            ⁢                                              ⅆ                        z                                                                                            ]                                              =                      A            ⁢                          xe2x80x83                        ⁢                          β              ⁡                              (                z                )                                      ⁢                          exp              ⁡                              [                                                      -                    2                                    ⁢                                      τ                    ⁡                                          (                                                                        z                          L                                                ,                        z                                            )                                                                      ]                                                                        (        1        )            
where A is an instrumental calibration constant, xcex2(z) is the 180xc2x0 backscatter coefficient (mxe2x88x921srxe2x88x921) from both molecules and aerosols at height z(m), "sgr"e is the extinction coefficient (mxe2x88x921) from both molecules and aerosols at height z, and xcfx84(zLz) is the extinction optical depth between the lidar height, zL, and z. Equation 1 shows that the fundamental challenge of converting the lidar measurement, S(z), to a geophysically meaningful aerosol quantity is to disentangle xcex2 and xcfx84xe2x80x94 or, equivalently, xcex2 and "sgr"e. Since molecular scattering can be predicted accurately from air density (i.e. temperature and pressure) information, this challenge reduces to disentangling particulate backscattering, xcex2p, from particulate extinction, "sgr"p. Two types of technologically advanced lidar systems, Raman lidar and high spectral resolution lidar, are able to separate these terms by making auxiliary measurements of the return signal. These instruments are described briefly below.
For lidar systems that detect elastically scattered light only, the quantities xcex2p and "sgr"ep can be disentangled if the ratio of the two parameters is known. This quantity is referred to as the lidar ratio, K,                               K          ⁡                      (            sr            )                          =                                            σ              ep                                      β              p                                =                                                    σ                sp                            +                              σ                ap                                                    β              p                                                          (        2        )            
where "sgr"sp and "sgr"ap are the components of particulate extinction due to light scattering and light absorption, respectively.
Based on Mie calculations that incorporate the ranges of particle size distributions and refractive indices encountered in the troposphere, possible values of K span at least an order of magnitude, from approximately 10 to 100 (sr). The lower values correspond to coarse-particle aerosols like soil .dust and sea salt, while the higher values represent fine particles of smoke and products of gas-to-particle conversion. To explore the sensitivity of lidar-retrieved optical depth to uncertainties in K, we use data from the recent lidar demonstration Shuttle mission (LITE). Table 1 shows the effect on retrieved optical depth of allowing K to vary from 10 to 100. Data consists of two cases when aerosol layers were detected at night over Africa during the LITE mission. The columns labeled ∂logxcfx84p/∂logK indicate how a fractional uncertainty in lidar ratio would translate into a fractional uncertainty in optical depth. This sensitivity parameter is seen to vary between the two cases and to be a strong function of lidar ratio. For low K values, K and xcfx84p are nearly proportional. For the higher K values (which tend to be characteristic of pollution-derived particles in the sub-xcexcm size range), the sensitivity is considerably higherxe2x80x94up to a factor of 4. Overall, the factor of ten range of possible lidar ratios translates into a factor of 10 to 40 uncertainty in retrieved optical depth. This range is too large to offer an adequate constraint on lidar retrievals for the problems of climate forcing or visibility.
For lack of accurate knowledge of K, most aerosol measurements by elastically scattered lidar are reported as a xe2x80x9cscattering ratioxe2x80x9d xe2x80x94 that is, the ratio of the calibrated signal to the expected signal for particle-free air. This term is useful for qualitative identification of aerosol layers, but not for input into radiative transfer models. The instrument described herein provides a relatively inexpensive method for accurate local measurement of xcex2p. When combined with existing instrumentation for measuring "sgr"ep, this permits an empirical determination of K.
Being small and portable, the new device permits routine ground-based monitoring as well as airborne surveys of xcex2p and K, which will, in turn, allow extensive lidar data sets on tropospheric aerosols to be applied in a quantitative fashion to the aerosol/climate and visibility problems.
The xe2x80x9cbackscattersondexe2x80x9d described by Rosen and Kjome In xe2x80x9cBackscatersonde: a New Instrument for Atmospheric Aerosol Research,xe2x80x9d Applied Optics, Vol. 30, pp. 1552-1561 (1991) offers a local measurement of xcex2p. The backscattersonde is light and inexpensive, and thus well suited for balloon-borne measurements of atmospheric backscatter versus altitude; in contrast, the instrument described herein is currently both too large and too expensive for routine balloon deployment. The backscattersonde has been used to determine the lidar ratio by running it in parallel with a separate instrument that measures scattering and with assumptions about particle absorption.
The backscattersonde has an open sensing volume and a flash lamp light source, so it cannot be calibrated in the laboratory with gases or with particles of known concentration, size and refractive index, and it can only be used at night. The calibrations rely on measurements of air Rayleigh backscattering in the stratosphere in the winter Arctic polar vortex, where particle concentrations are believed to be insignificant. Previous or subsequent measurements in other regions rely on inter-instrument calibration via comparison to reference instruments. However, optical and electronic components may be subject to drift and the resulting uncertainty has not been determined. The instrument senses backscattering over a broad angular range (xcx9c160xc2x0-179xc2x0) and over two broad wavelength ranges centered at 490nm and 700 nm, with bandwidths of about 100 nm. The backscatter at 532 nm is derived by linear interpolation. For these reasons, even for a calibrated system, converting the measured quantity to xcex2p at 532 nm would require an optical model of the instrument and Mie calculations based on assumptions about particle size, refractive index, and sphericity. Thus, the backscattersonde offers a proxy for xcex2p at 532 nm that requires calculations and assumptions that would preferably not be required in an ideal system.
Another technique relevant to measuring aerosol scattering is the Raman lidar method. Molecular and aerosol contributions to light extinction are separated in this. method by measuring Raman-shifted laser light at the appropriate wavelengths for nitrogen, oxygen, carbon dioxide and/or water vapor. Laser light that has been elastically scattered by both molecules and aerosols is also measured. The intensity of the Raman-shifted backscatter from a given altitude depends on "sgr"sg(z), "sgr"sp(z), and on xcex2gas(z), but not on xcex2p(z). The terms "sgr"sg(Z) and xcex2gas(Z) can be calculated, given an assumed or measured (with a radiosonde) atmospheric density, so inversion yields "sgr"sp(z) at the Raman-shifted wavelength. Aerosol extinction at the original laser wavelength is determined from the Raman-shifted signal by using an assumed wavelength-dependence of light scattering, which is based on an assumed size distribution.
To date, this technique has mostly been applied to ultra-violet wavelengths. Because of the strong wavelength dependence of the lidar ratio (according to Mie calculations) for particles below about 10 xcexcm, lidar ratios measured at ultraviolet (UV) wavelengths with Raman lidar are not directly applicable to visible-wavelength lidar. Conversion to visible wavelengths requires use of an aerosol model (essentially, an assumed aerosol size distribution) that can introduce uncertainties of a factor of two. High spectral resolution lidar (HSRL), like Raman lidar, solves the lidar inversion problem by separating the backscattered light into particulate and molecular components. HSRL takes advantage of the fact that molecules in the atmosphere have much greater Brownian motion than particles, so backscattered light from molecules is wavelength-broadened around the original laser wavelength. An interferometer is used to measure this broadened molecular backscatter. As with the Raman-shifted backscatter described above, the molecular return signal depends on total extinction and gaseous backscatter only, so "sgr"sg(Z) can be determined directly, given "sgr"sg(Z) and xcex2gas(z).
Both the Raman lidar and HSRL are quite expensive and technologically complex. Like other remote or open-air devices (including the backscattersonde), they cannot be calibrated with laboratory particles of known optical properties, and the absolute accuracy of their inversion is difficult to quantify. Independent verification of the measured optical properties is therefore useful. On the other hand, these open-air devices have the enormous advantage of measuring the undisturbed ambient aerosol and can be used to explore vertical variations in the lidar ratio and its sensitivity to ambient relative humidity.
Retrieval of aerosol optical parameters from lidar systems without Raman capability is also possible, given certain assumptions and/or coincident measurements by other instruments. Sun photometers are often used to measure total column optical depth (xcfx84) for vertically pointing lidars. Generally, xcfx84 must be wavelength corrected to the given lidar wavelength. In addition, xcfx84 is measured for the entire atmosphere, whereas the lidar measurement is only over a portion of the atmosphere (zL-z in Equation 1). One approach is to assume that above z the atmosphere is aerosol-free and use a fixed lidar ratio to determine aerosol extinction from the lidar and sun photometer data alone. The sun photometer has been used in conjunction with an optical particle counter (OPC), which determines the ground-level aerosol size distribution for an assumed index of refraction. The aerosol extinction and backscatterxe2x80x94and thus the lidar ratioxe2x80x94are calculated from Mie theory and the OPC data. In these calculations, aerosol optical properties have been considered to be horizontally and vertically homogeneous. It has been assumed that the return signal from the stratosphere is aerosol-free and so, is usable for lidar absolute calibration. However, it is recognized that this approach is invalid after significant volcanic eruptions, because the assumption of an aerosol-free stratosphere is violated.
Another approach employs an aureolemeter, which views forward-scattered sunlight, in conjunction with the sun photometer. The aureolometer gives a columnar averaged size distribution, assuming spherical aerosols with a given index of refraction; this information is useful for Mie calculations of aerosol scattering. An advantage of this method is that forward-scattered radiation is not as sensitive to shape and index of refraction as it is to size, so error in the assumed input parameters is not likely to significantly corrupt the derived size distribution.
Bistatic lidars measure scattered light at a range of angles, providing information on the phase function of the column-averaged aerosol. This data can be used to determine the most probable aerosol index of refraction and size distribution. Mie calculations are then employed to perform the lidar inversion. A ground-based nephelometer has been used with a vertically pointing lidar and calculated lidar ratios by assuming no light absorption and vertical homogeneity of the aerosol. A ground-based nephelometer has been used in conjunction with a bistatic lidar to calculate lidar ratios by assuming no light absorption and vertical homogeneity of the aerosol. Bistatic lidar data have also been inverted using Mie theory with an assumed aerosol size distribution and refractive indices.
Several groups have made measurements of aerosol optical properties in the boundary layer using horizontally. pointing lidars. This measurement is a somewhat easier retrieval problem, in that assumptions of aerosol homogeneity over the lidar optical path are more likely to be accurate. A hard target with fixed optical properties can be used to calibrate a horizontally pointing lidar. Optical properties for a generated aerosol of high optical depth can then be derived using the calibrated lidar. However, the generated particles may not be representative of real atmospheric aerosols. To measure ambient atmospheric aerosols, one group used meteorological data from a lidar site to calculate molecular scattering and an Active Scattering Aerosol Spectrometer Probe (ASASP) to determine the aerosol size distribution at the site. In horizontally homogeneous conditions (i.e., when the lidar signal decreased linearly with range), the lidar ratio could be derived from this data alone. Another group used filter sampling methods and an optical particle counter to determine aerosol size and refractive index, then calculated lidar ratios using Mie theory. Assuming horizontal homogeneity, the system calibration was complete.
All of these approaches to lidar data retrieval require some combination of additional measurements and assumptions about the physical properties of the aerosols. Mie theory is almost always employed to connect these properties to light scattering characteristics, which must be known to retrieve physically meaningful data from the lidar signal. However, Mie theory may inaccurately represent the optical properties of the aerosols, especially if they are non-spherical, even if the input parameters are correct. It would be preferable to employ a direct determination of an aerosol""s lidar ratio, to eliminate the need for measurements and assumptions about particle physical properties and subsequent calculation of optical properties and thereby, to improve the accuracy of the result.
In accord with the present invention, apparatus for measuring a backscatter coefficient for an aerosol entrained in a fluid is defined. The apparatus includes a housing defining a chamber having an inlet port through which a fluid enters the chamber, and an outlet port through which the fluid exits the chamber. An interior of the housing is covered with an optically light absorptive coating that provides minimal light reflection. Disposed within the housing is a light detector, which produces an output signal indicative of an intensity of light incident on the light detector. A plurality of light baffles are disposed between the light detector and the chamber and an optical sensing path of limited scope extending from within the chamber to the light detector. Only light traveling along this optical sensing path toward the light detector is detected by the light detector. A coherent light source produces a beam of coherent light that is directed away from the light detector at an acute angle relative to a central axis of the optical sensing path. The coherent light is reflected from the aerosol within the chamber and the reflected light travels back along the optical sensing path and is detected by the light sensor. Accordingly, the signal produced by the light detector in response to this reflected light is indicative of the backscatter coefficient for the aerosol. Preferably, the acute angle is less than 4xc2x0, and more preferably, less than 2xc2x0.
The apparatus also preferably includes a rotating chopper disk that has a plurality of sectors with different optical properties, and the sectors are selectively positionable in the optical sensing path. The sectors include an open sector through which light freely travels, a light absorbing sector that blocks and absorbs substantially all light incident upon it, and a calibration sector that transmits only a predefined, relatively small portion of the light incident upon it, diffusing the light that is transmitted over a large area so that only a portion travels along the optical sensing path.
A partially reflecting surface disposed to reflect a portion of the coherent light produced by the coherent light source along a different path than a remainder of the coherent light forms a reference beam. The reference beam is directed toward the chopper disk at an angle so that the reference beam does not travel directly along the optical sensing path toward the light detector when passing through the open sector. When the reference beam is directed at the calibration sector, the diffusion and limited transmission of the reference beam through the calibration sector ensure that only a portion of the reference beam travels toward the light detector along the optical sensing path. This small portion of the reference beam that is detected by the light detector causes the. light detector to produce a signal that is indicative of an intensity of the coherent light source, which is used to compensate for variations in the intensity when determining the backscatter coefficient of the aerosol.
Also included in the apparatus is at least one baffle, disposed within the chamber to prevent light reflected from surfaces of the housing traveling along the optical sensing path toward the light detector. The beam of coherent light is directed toward a light absorption surface, which is disposed within the chamber, so that the beam strikes the light absorption surface at a point outside the optical sensing path, thereby minimizing any reflected light from this surface traveling back toward the light detector along the optical sensing path.
In one embodiment, the apparatus includes a non-coherent light source that is selectively enabled to transmit light into the chamber in a direction generally transverse to the optical sensing path during a time interval when the coherent light source is selectively. disabled from transmitting coherent light into the chamber. Light produced by the non-coherent light source that is reflected from the aerosol toward the light detector along the optical sensing path causes the light detector to produce a signal indicative of light extinction due to total light scattering by the aerosol. In this embodiment, electronically controlled shutters can be disposed between the chamber and both the coherent and non-coherent light sources, to selectively block light produced by the coherent light source and the non-coherent light source during alternate time intervals.
Another aspect of the present invention is directed to a method for measuring a lidar ratio of an aerosol entrained in a fluid. The method includes the step of providing a light detector in a housing through which the fluid and the aerosol are circulated, and a coherent light source. A beam of coherent light produced by the coherent light source is transmitted substantially along an optical sensing path of the light detector, but directed away from the light detector, along a path that deviates from a central axis of the optical sensing path by only a small acute angle. Light scattered by the aerosolparticles back along the optical sensing path is detected by the light detector, producing a signal determinative of light backscatter from the aerosol, at about 180xc2x0. In addition, light extinction due to total light scattering by the aerosol is measured, as well as the light absorption of the aerosol. The lidar ratio of the aerosol is then determined as a function of the light backscatter, the light extinction due to total light scattering, and the light absorption by the aerosol.
The lidar ratio is equal to the sum of the light extinction due to scattering and the light absorption of the aerosol divided by the light backscatter of the aerosol.
The small acute angle by which the beam of coherent light deviates from the central axis of the optical sensing path is preferably less than 4xc2x0, and more preferably less than 2xc2x0. Thus, the signal produced by the light detector is substantially indicative of 180xc2x0 light backscattering for the aerosol.